A photocurable resin composition contains a component to be polymerized (for example, a monomer or an oligomer) and a photopolymerization initiator. The irradiating light for curing the photocurable resin composition is typically ultraviolet light and the like. The photocurable resin composition may be used as an adhesive, and is used for a wide application such as, for example, temporary stoppage of components to a print substrate, substrate masking, a main seal of a liquid crystal panel even in the field of electronic materials and record display materials.
As mentioned above, the photocurable resin composition may be used as an adhesive. In recent years, in the field of electronic materials and record display materials, the photocurable resin composition has been required to have high sensitivity, a low out-gas property and a high adhesion strength to a substrate. In addition, a photocurable resin composition, which is used as a sealing agent of a flat panel display including a liquid crystal display panel and an organic EL display panel, has been required to be cured by ultraviolet light in a relatively long wavelength region or visible light.
In the production of a liquid crystal display panel by the liquid crystal “one-drop-fill” (ODF) process, a liquid crystal is dropwise added to a substrate on which a sealing agent composed of a photocurable resin composition is applied and followed by light irradiating to cure the sealing agent. If the irradiating light is ultraviolet light of a short wavelength region, the liquid crystal molecules and the like are frequently damaged. For this reason, a special consideration has been required for light-shielding parts except for the desired irradiation part, narrowing the spot radius of a light source, and the like. Therefore, the photocurable resin composition for the sealing agent is strongly required to have a property to be cured by ultraviolet light in a relatively long wavelength region or visible light.
Furthermore, if a irradiating light for curing a photocurable resin composition is transmitted through an organic material substrate such as polycarbonate and the like, ultraviolet light having a short wavelength of less than 370 nm is absorbed by the substrate, thereby causing a problem that the resin composition was difficult to be sufficiently cured.
On the contrary, since ultraviolet light having a wavelength of 370 nm or more and visible light have a high transmissivity for an organic material substrate and there can be used any well-known lamp light source such as a high-pressure mercury lamp, a low-pressure mercury lamp, a metal halide lamp, a xenon lamp, a fluorescent lamp and the like, they are strongly required to be used as a wavelength for photocuring, from the viewpoint of environmental burden and energy saving.
In order to cure a photocurable resin composition by ultraviolet light in a relatively long wavelength region and visible light, it is important to suitably select a photopolymerization initiator. The conventional photopolymerization initiator is roughly classified into a self-cleavage type photoinitiator, a hydrogen-extraction type photoinitiator, and a photoinitiator using a light-induced electron transfer reaction.
The self-cleavage type photoinitiator is represented by an acetophenone derivative or the like, which absorbs light energy and is then cleaved to active radical species to initiate polymerization reaction. The hydrogen-extraction type photoinitiator is represented by benzophenone, thioxanthone and the like, which absorbs light energy and then generates active radical species by the hydrogen-extraction reaction of a carbonyl group to initiate polymerization reaction. The photoinitiator using a light-induced electron transfer reaction is represented by a combination of triazine and an organic dye, or a combination of an organized oxide and an organic dye, and the like.
Most of the photoinitiators contained in a common photocurable resin composition react with ultraviolet light. The photocurable resin composition containing such photoinitiators is not cured, or a sufficiently cured product may not be obtained without irradiating with the ultraviolet light having a relatively short wavelength of 300 nm or more and less than 370 nm.
On the one hand, as a photoinitiator reacting with a visible light, there is reported a photoinitiator which is a combination of thioxanthones, amines, camphorquinone, and the like (refer to Patent Document 1). The photoinitiator is not completely consumed by light irradiation and an unreacted photoinitiator remains. The unreacted photoinitiator may be eluted from the cured product or sublimated. In the food field where safety is emphasized and in the application of electronic components in which a small amount of impurities causes a problem, even if the generation amount of the photodegraded product is small, the unreacted photoinitiator may be transferred to the surface of the cured product to cause a problem. The malodor of amines may also cause a problem.
In addition, there has been developed a photocurable resin composition containing an α-aminoacetophenone-based or acylphosphine oxide-based photoinitiator, which is a self-cleavage type photopolymerization initiator having a light absorbance from ultraviolet light of a long wavelength region to visible light. However, since, for example, these photoinitiators generate, for example, benzaldehyde as a photodecomposed product, they could not be said to be a preferable photoinitiator from the viewpoint of reducing the amount of out-gas such as, for example, VOC (a volatile organic compound) and the like.
There has been proposed a photosensitizing agent having a (meth)acryloyl group at a part of a tetraalkyl-diamino-benzophenone skeleton (refer to Patent Document 2). However, the synthesis method is complicated, the radical generation efficiency is insufficient and the sensitivity is poor. There has been also proposed a thioxanthone derivative having an ethylenic unsaturated group (refer to Patent Document 3), but the electronic density of the aromatic ring is decreased because a carbonyl group is directly introduced into the thioxanthone as a substituent. For this reason, the radical generation ability is decreased and the sensitivity may be insufficient.
In addition, there has been reported a photopolymerizable substance, which contains a compound having a thioanthraquinone skeleton, for producing a relief printing block (refer to Patent Document 4).
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-313216
[Patent Document 2] Japanese Patent Laid-Open Publication No. Hei 6-211758
[Patent Document 3] Japanese Patent Laid-Open Publication No. 2004-224993
[Patent Document 4] Japanese Patent Laid-Open Publication No. Sho 53-142490